276 alsberg: biochemical analysis of nutrition 



the defective proteins lack one or more of the amino-acids which 

 are found in the proteins that are not defective. This is very 

 much oftener true for the vegetable proteins than for the animal 

 proteins. Some lack lysine, others tryptophane or histidine, or 

 cystine. The latter is an amino-acid containing sulphur, the 

 usual form in which sulphur is contained in proteins. Some 

 proteins lack more than one amino-acid. Gelatine, for example, 

 contains no cystine, tyrosine, or tryptophane. Now it has been 

 shown in certain cases that if to a diet of the t kind just described, 

 containing a single defective protein, there be added the amino- 

 acids which that protein lacks, the value of the diet is greatly 

 increased; in certain instances it may even become entirely 

 capable of supporting life and growth. We have here a direct 

 proof that the animal organism is capable of utilizing amino- 

 acids and incapable of manufacturing for itself certain amino- 

 acids. Herein it differs from the plant organism which is cap- 

 able of making all the amino-acids necessary to support its life. 

 The animal organism is, however, capable of making certain 

 amino-acids. It can, for example, make glycine. It has not 

 as yet been finally determined exactly which amino-acids can 

 be made by animals and which can not. 



There are a number of ways in which the lack of certain 

 amino-acids may affect the functioning of the animal organism. 

 Their lack may, of course, make it impossible for the animal 

 to manufacture its own tissue protein. It suffers a kind of 

 starvation. There are, however, more indirect ways in which 

 the absence from the diet of a necessary amino-acid may be 

 important. It has recently been found that the iodine com- 

 pound of the thyroid gland, the gland that you feel in the neck 

 about the Adam's apple, is a derivative of the amino-acid trypto- 

 phane. It has long been known that the normal functioning 

 of the thyroid gland is essential to life and health. It has 

 been found that the normally functioning gland contains the 

 iodine compound now believed to be formed from tryptophane. 

 It is therefore possible that when there is no tryptophane in the 

 diet, difficulty in the formation of the iodine compound necessary 

 for the thyroid gland results with corresponding disturbance 

 of the gland's function. 



